Last week during game one of the NBA Finals between the Miami Heat and San Antonio Spurs, the air conditioning system in the AT&T Center wasn’t working properly. As a result, the normally cool arena was a sweltering 90 degrees fahrenheit (~30 degrees Celsius). Miami’s LeBron James suffered a debilitating bout of muscle cramps, and had to leave the game. LeBron James is the single most dominant basketball player of his generation. So when he misses a significant portion of one of the most important games of the year, it is a Big Deal. Miami went on to lose the game 110 to 95.

Almost immediately, commentators went into a frenzy discussing how the heat and related dehydration/salt loss caused King James’ cramps (in the above video, the heat is mentioned several times within seconds of James cramping up). This article from ESPN, was pretty typical of the coverage that I saw online:

Miami Heat forward LeBron James was forced to exit early from Game 1 of the NBA Finals because of severe leg cramping caused by extremely warm temperatures after the air conditioning in the arena malfunctioned.

“In a regular game, professional athletes lose an extraordinary amount of fluid and electrolytes,” says Dr Michael Bergeron, executive director of the Sanford Sports Science Institute. “Playing in hot and humid conditions can push a player’s fluid and electrolyte loss to a dangerous level.” As dehydration sets in, subtle twitches or cramping can progressively turn into painful muscle spasms.

The prevailing wisdom being that the heat caused dehydration and/or electrolyte loss, which caused his muscle cramps. The analysis has been fast and furious (the classiest was Gatorade pointing out on Twitter that people who consume their sugar water sports drink can “take the heat” – LeBron is sponsored by Powerade), but most of it seems to miss a crucially important point: research suggests that exercise-related muscle cramps are not caused by dehydration or electrolyte loss (I was surprised that even articles explaining “the science of muscle cramps” largely sidestepped the issue).

The real explanation is way less sexy: exercise induced muscle cramps are caused by plain old fatigue.

The idea that muscle cramps are caused by dehydration first originated from anecdotal reports of miners suffering from “heat cramps”. The miners were doing a very physically demanding job in a very hot environment, and therefore sweating a lot. Based on this information, it would be reasonable to hypothesize 2 different potential causes of their cramps:

Dehydration and/or salt loss through high levels of sweating

Muscle fatigue, from their physically demanding work

Essentially what happened is that people latched onto hypothesis # 1, and never thought to investigate hypothesis # 2 (until recently at least). And since exercise-related cramps often occur when people exercise in hot environments (e.g. LeBron’s situation), it seemed to support the idea that dehydration and/or salt loss were to blame.

However, that evidence is based almost entirely on anecdotal reports, rather than high quality research. A fantastic review in the British Journal of Sports Medicine looks at the available research evidence, and argues that dehydration and salt loss are unlikely to have anything to do with exercise-induced muscle cramps.

For example, several recent studies have examined electrolyte levels among crampers and non-crampers following endurance events. They found no differences in electrolyte concentrations between the two groups, and electrolyte levels did not change when the cramps disappeared. Similarly, cramping athletes are not any more dehydrated than their non-cramping counterparts. It’s also worth noting that “heat cramps” have been observed in cool environments, such as swimming in cold water (conversely, cooling hot a person does not make their cramps go away). Core temperature does not seem related to cramping either.

Some researchers have suggested that “salty sweating” (e.g. losing large amounts of salt through in sweat) could cause cramping, without any change in overall electrolyte concentrations in the blood. This hypothesis is based on a study that found that athletes who self-identify as crampers had higher levels of sodium in their sweat than control athletes who did not identify as crampers (it’s important to note that they didn’t measure sweat DURING a cramp – these were simply athletes with a history of cramping). However, the cramp-prone athletes still had relatively low levels of salt in their sweat when compared to other studies that examined sweating in a wide range of non-cramping individuals. In other words, these “salty sweaters” didn’t have all that much salt in their sweat when compared to other groups of non-crampers.

It is worth noting that systemic (e.g. whole body) reductions in sodium concentrations can result in muscle cramps. This can sometimes happen during hemodialysis. However, this results in generalized cramping – e.g. cramps that occur throughout the body. Whereas muscle cramps are localized – they typically occur in only one or two muscles at a time (the calves, feet and hamstrings are especially prone to cramping). This fact alone – that only the muscles being exercised are prone to cramping – is a pretty big indication to me that cramps have more to do with muscle fatigue than with whole body electrolyte concentrations.

What evidence links fatigue with muscle cramps?

Observational studies have found that athletes often report muscle fatigue prior to the onset of cramps themselves. They have also found that athletes are more likely to experience cramps when they identify as being under-trained for the event, or as competing at a higher intensity than during training. For example, one prospective study asked 210 Ironman athletes to complete a detailed history before a race. 44 of them developed muscle cramps, and researchers found that when they matched crampers with non-crampers that had similar training and performance histories (e.g. coming into the race, you would expect the two athletes to have a similar performance), those who developed cramps had also chosen faster pacing strategies. In other words, the people who raced at a pace that was too fast for their ability were also more likely to develop cramps at some point in the race.

Another piece of evidence suggesting that cramps are caused by muscle fatigue is that numerous studies have shown that voluntary muscle contractions can be used to cause muscle cramps! Further, it is easier to induce muscle cramps following a hard workout, than it is before a hard workout. This alone is a pretty big indication that muscle cramps are more likely to be due to localized muscle fatigue, rather than systemic changes in hydration or salt concentrations.

Finally, the last piece of evidence suggesting that cramps are due to fatigue is that passive stretching helps to end a cramp. Dr Schwellnus (the author of the BJSM review) suggests that this could be explained by stretch receptors in your tendons called Golgi tendon organs, that somehow become less active following a bout of fatiguing exercise. These receptors sense stretch in your tendons, and when they sense too much stretch, they basically shut down your muscle to reduce strain and prevent injury (e.g. if the muscle stops contracting, the tension in the tendons will be reduced, and you won’t rip your muscle and/or tendon). They essentially work like brakes that prevent unnecessary or potentially dangerous muscle contractions.

The hypothesis is that when you are fatigued, these Golgi tendon organs become less active, so there is less inhibition of muscle activity. Without this inhibition from the Golgi tendon organs, your muscles are more likely to contract on their own. Your muscle is now like a car with brakes that aren’t working at full capacity.

This fits with the fact that contractions are more likely to occur when a muscle is already in a shortened position (e.g. a cramp is more likely in your hamstrings if you pull your feet up towards your butt) – this is when the activity of the Golgi tendon organs is likely to be very low, since there is very little stretch in the tendons. This would make it easier for your muscle to spontaneously contract, since it’s getting less inhibition than it would in the non-fatigued state. It also explains why stretching can help to end a cramp – this increases the activity in the Golgi tendon organs, which helps convince the muscle to stop contracting.

What does this mean for LeBron?

LeBron’s cramps last week seem to fit with the general pattern of exercise-induced muscle cramps. They came during a very hard physical effort, at the end of a long season and playoff run. They went away briefly when he went to the bench, then returned when he went back on the court (if they were caused by dehydration or salt loss, it would seem strange that they would go away with rest, only to return when he started to exert himself again). The good news is that a bit of rest seems to do the trick.

LeBron is one of the best players in the world right now, so it’s in the Heat’s best interest to use him as much as possible. They just have to avoid using him so much that his muscles are fatigued to the point where they cramp up. The trick is to get as close to that line as they can without going over. And it’s worth mentioning that exposure to high heat may make muscle cramps more likely (there’s not great evidence on this one way or another), but eating extra salt is unlikely to help prevent cramps from happening.

What is the take-home message?

There is still a lot we don’t know about exercise-induced muscle cramps.

What we do know:

they typically occur after strenuous exercise

they can be caused by voluntary muscle contractions

they are localized to the exercising muscles

they are more likely when the muscle is in a shortened position

they go away with passive stretching and rest

If, like me, you’ve had muscle cramps in the past, think about how they fit with the above criteria. I once had a massive cramp in my hamstring while on a first date in high school – it was extremely painful in more ways than one. Not coincidentally, it was a couple of weeks before a major race, and I had finished my hardest track workout of the season just an hour or two before. In fact, the only time I’ve ever had muscle cramps has been in the hours following very hard workouts. And they have always gone away as soon as I’ve been able to stretch and rest the offending muscle (one of the few instances where I’m a big fan of stretching!).

I’ve leaned heavily on the review paper by Schwellnus for this post, so I’d highly recommend you go check it out. A full version of the paper can be found here.

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Travis Saunders has a PhD in Human Kinetics, and is currently an Assistant Professor in Applied Human Science. His research focuses on the relationship between sedentary time (e.g. sitting) and chronic disease risk in both children and adults. He is also a Certified Exercise Physiologist and (former) competitive distance runner. You can connect with him on Twitter @TravisSaunders.

8 comments

I’ve also had cramps after workouts, especially after swimming (which I don’t do very often), so that fits, but I also have cramps in the middle of the night and now that I am pregnant, I have cramps much more often than normally. I know that this is common in pregnant women. Any idea why that could be?

My wife has also had more muscle cramps since becoming pregnant, and I’ve personally just assumed that it’s due to the extra strain of carrying around the baby (if I had to carry an extra 20-30% of my body weight for several months, I could see that resulting in a lot of muscle fatigue!).

“They essentially work like breaks that prevent unnecessary or potentially dangerous muscle contractions.” I hope that should that be “brakes” (and in the next quote).

“The hypothesis is that when you are fatigued, these Golgi tendon organs become less active, so there is less inhibition of muscle activity. Without this inhibition from the Golgi tendon organs, your muscles are more likely to contract on their own. Your muscle is now like a car with breaks that aren’t working at full capacity.”

That’s partly right – and it often happens after a harder than usual workout. Cramps happen when exactly? When you extend the joint, e.g. when you point your toes, your calf muscle may cramp. So for example someone who has cycled at a higher intensity or for longer than usual – their calf muscles have been operating over a short range of motion for very many multiple cycles (sorry about the pun) probably rapidly (e.g. a cadence of 120) and against a high resistance (e.g. 240 W). When they dismount and as soon as they extend that range, the calf muscle “cramps” up – It is simply trying to do the same as it was a few moments ago in maintaining a short range of motion – but now there’s no resistance and it is not constrained.

I think we’re saying the same thing – if you have a cramp in a muscle (e.g. the calf), then stretch that muscle and the cramp will likely go away (to be clear, I’m not talking about stretching out the full leg, but rather the specific muscle that is cramping/likely to cramp).

Thanks for the great insights. My leg cramps are always worse after zumba–but only if the class is at night–but they don’t manifest themselves until I go to bed. (Talk about perverse.) If I go to a morning class, I don’t have any problems. What accounts for that?

I have also noticed that once I hit peri-menopause, leg and foot cramps (especially agonizing) were much more frequent. Any idea why? Hormones, maybe?

That would be my guess, although I’ll admit that it’s a very uneducated one! I’m wondering if when you do Zumba at night, your legs may already be tired from the day? Again though, I’m really not sure. I have found that personally I get more cramps after evening workouts as well.

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